Dimerization of olefins is well known and industrially useful. Further, the use of transition metals to catalyze olefin dimerization and oligomerization is also known.
Use of ionic liquids for dimerization and oligomerization of olefins is also well known. In the broad sense, the term ionic liquids includes all molten salts, for instance, sodium chloride at temperatures higher than 800° C. Today, however, the term “ionic liquid” is commonly used for salts whose melting point is relatively low (below about 100° C.). One of the earlier known room temperature ionic liquids was [EtNH3]+[NO3] (m.p. 12° C.), the synthesis of which was published in 1914. Much later, series of ionic liquids based on mixtures of 1,3-dialkylimidazolium or 1-alkylpyridinium halides and trihalogenoaluminates, initially developed for use as electrolytes, were to follow.
One property of the imidazolium halogenoaluminate salts was that they were tuneable, i.e., viscosity, melting point and the acidity of the melt could be adjusted by changing the alkyl substituents and the ratio of imidazolium or pyridinium halide to halogenoaluminate. Imidazolium halogenoaluminate salts exhibit moisture sensitivity and, depending on the ratio of aluminum halide, Lewis acidic or Lewis basic properties. Ionic liquids with ‘neutral’, weakly coordinating anions such as hexafluorophosphate ([PF6]−) and tetrafluoroborate ([BF4]−) have also been used as alternatives to imidazolium halogenoaluminate salts. [PF6]− and [BF4]− based ionic liquids are generally highly toxic. Yet another anion for use in ionic liquids is bistriflimide [(CF3SO2)2N]−, which does not exhibit the toxicity of [PF6]− and [BF4]− anions. Ionic liquids with less toxic cations are also known, including those with compounds like ammonium salts (such as choline) being used in lieu of imidazole.
Ionic liquids have found use as a catalyst in various chemical reactions. For example, Lewis acidic ionic liquids have been used as a catalyst to alkylate aromatic hydrocarbons, such as the alkylation of benzene with ethylene. In such processes, the ionic liquid itself serves as the catalyst, and the catalyst is neither buffered nor immobilized on a support. Ionic liquids have also been used in processes for making high viscosity polyalphaolefins using an oligomerization catalyst including an aluminum halide or alky-aluminum halides, and alkyl-substituted imidazolium halide or pyridinium halide. In such processes, the ionic liquid itself again serves as the catalyst and preferentially forms high-viscosity polyalphaolefins due to the lack of buffering.
Processes utilizing immobilized ionic liquids are also known. For example, immobilized ionic liquids may be prepared by functionalizing a support prior to contact with or forming the ionic liquid. Such known immobilized ionic liquids however are not buffered and therefore preferentially form high viscosity polyolefins. Again, in such systems, the ionic liquid itself functions as the catalyst.
Although all of the above methods are known and used in the synthesis of olefins, what is needed in the art is an improved synthetic method that allows for easy separation of the product. Especially in the case of olefin dimerizations, which usually yield liquids with relatively low viscosities or even gaseous olefins, the application of supported systems that allows the use of fixed bed reactors is superior to batch oligomerization, obviating the need for further product separation. In addition, the catalytically active surface may be maximized by use of high surface support materials, which optimizes the catalytic performance.